In paper and paperboard making processes there is a desire to obtain strong products with high bulk (low density). Paper and paperboard products having high bulk require less fiber amounts, which is desirable especially for economical reasons. Also, bulky paperboard products with low fiber content have a better bending stiffness which also is desirable. Paper and paperboard products are typically produced by dewatering furnish on a wire. The furnish often contains a mixture of different pulps, including both chemical pulps, mechanical and/or chemimechanical (CTMP) pulps. In order to produce a bulkier sheet with higher structural stiffness the interest in using mechanical or CTMP pulps with high freeness has increased. To obtain the best result the mechanical or CTMP pulp should contain long, intact fibers and as little fine material as possible. However, pulps that are rich in stiff, long fibers unfortunately show poor ability to produce sufficient fiber bonding of the paper or board. The bulk and strength properties of the resulting paper or board will therefore be a compromise between the ability of the pulp to increase the bulk and its ability to increase the fiber bonding properties of the paper or board.
The fiber bonding properties of mechanical or CTMP pulps may also be improved by treatment with chemical additives. The predominant treatment for improving strength, particularly dry strength, of paper or board has so far been to add cationic starch to the pulp fiber slurry prior to the sheet forming operation. It is however difficult to adsorb large amounts of starch to the fibers, especially when the fines amount is small. One way to increase the amount of starch retained in a paper or paperboard products is to treat the fibers with polymers in several steps as is described in WO0032702 A1 and WO 2006041401 A1.
However, there is still a need for a method for producing paper or paperboard, which has an improved strength without negatively affecting the bulk.